Radiometer

[Dan M]

I searched on this site and found nothing about radiometers.

(black and white solar powered spinny things to be technical)

Has anyone (or is there any point) looked at making a REALLY big one as a RE project?

I don't know how to calculate the theoretical limits on the torque for something like this.  It might be something the size of a school bus that puts out 10-20 watts.

-Dan M

[jacquesm]

less than that, probably nothing at all !

you have to run them in a vacuum to get them to move at all, they can not overcome the drag of the air... Any kind of 'drive' would knock it out right away. Before you'd have any appreciable rotation (due to the inertia in your machine that size) you'd already be in the dark again. Magnetic bearing will help a bit here (It'd better be a passive one or there goes your power)

Check out the pages on 'solar sails' in space and the kind of momentum they can make:

(homepage is in French, click english flag) solar sail homepage

that's without an atmosphere, so an absolute best case !

[finnsawyer]

Not likely.  Light pressure can change the orbits of asteroids due to their spin, but it takes a long long time.

[troy]

Large transparent pressure vessels that can withstand the vacuum become geometrically more expensive with diameter increases.  At least, that's my SWAG (Scientific Wild Ass Guess).

Hmmmm, I could see long vertical tubes, like fluoresent light bulbs, with dozens or hundreds of vanes all on the same axle running on one very efficient bearing.

Have a lovely day,

troy

[Dan M]

There is a popular mis-conception about radiometers.

They do not run in a vacuum.

Here's the wrong explaination:

The black side "absorbs" em energy particles (mostly visible light) and the light side reflects them.  The reason it moves is the difference in energy imparted between elastic and in-elastic collisions.

This seems intuitively obvious, but if you watch a radiometer (or read the little pamphlet that comes with it) you will see that according to the above explanation IT'S SPINNING THE WRONG DIRECTION.  The black face is on the trailing side of each "paddle".

The right explaination:

The AIR immediately adjacent to the dark side is heated.  This local volume of AIR expands and becomes more active.  The higher energy collisions push on the black side more than the lower energy collisions on the white side.  This makes it spin with the white faces leading and the black faces trailing.

There's a notable shortage of AIR in a vacuum (sorry, sarcasm doesn't play well in text).

Now that being said, it may still require a partial vacuum.  I know these things sure look like a light bulb.

Anybody know if they pop when they break?

I wonder if filling the container with 1 atm of helium would accomplish the same thing as a partial vacuum?  Hydrogen might even work better, and it would be easier to destroy the evidence if the whole thing turns out to be a waste of time.

-Dan M

[Dan M]

OK I give.

I started looking, found a couple wrong explanations, more right ones, and several right ones that de-bunked the wrong ones.

HOWEVER:

I also found out how much (or how little) pressure is inside a radiometer.

They are evacuated to slightly less than 60 milli-torr absolute pressure, or 60 x 10e-9 m of mercury.

At about 0.76m to an atm, this leaves an absolute pressure of about .00000116 psi absolute pressure.

We all know what this means:

THERE ARE AIR MOLECULES IN THERE. (ok, so I can count them on one hand, they're still in there....-whimper-).

Bummer, I was picturing cars running around with great big glass bulbs on top of them.

-Dan M

[jacquesm]

hm, not so sure which of the two is the right explanation after reading this, I think it comes down to how low the pressure in a particular specimen is as to how it works.

How stuff works is usually pretty good at researching their stuff (even if overly commercial)

[Dan M]

I made a math or metric prefix error.

60 milli-torr is actually 60 x 10^-6 m of mercury or .00116 psi absolute.

and we all know what this means:

THERE'S A THOUSAND TIMES MORE AIR IN THERE THAN I THOUGHT THERE WAS 5 MINUTES AGO!

Forget powering cars, I'm going to go find an old school bus.

-Dan M

[jacquesm]

I should probably refresh the page just before posting after researching something

[DanB]

I think you got it right the first time.

A radiometer will not work in a complete vacuum.  They turn so that the white side is the 'leading edge' - if it had to do with "reflecting" photons and absorbing them..or anything like that - theyd go the other way.

Its the fact that the black side aborbs 'light' and warms up the air on that side that makes them go 'round I think.

They'll go really fast if you put them near a nice hot wood stove...

[jacquesm]

>Its the fact that the black side aborbs 'light' and warms up the air on that side that makes them go 'round

Appearantly they can spin BOTH ways depending on how good the vacuum is.

[JW]

I promised myself I wouldnt comment anymore today, so much for that. You guys better stop it, Im starting to re-ponder MHD on the leading edges of aircraft.

[jacquesm]

MHD ?

[JW]

Uhum crught hum ahum,

 Do a search on fieldlines for MHD...

was that a lung oyster?

-JW

[jacquesm]

it was used so offhand that I figured I must be dense... I heard of these things but never saw one explained. Thanks !